A fiber cord, particularly a fiber cord treated with an adhesive, are widely used as a reinforcement for the rubber products such as tires, conveyor belts, V-belts, hoses and so on. The fiber cord may be made of nylon fibers, polyester fibers, rayon fibers and the likes. Among the important methods for improving the performance of the final rubber products is to improve the physical properties of the fiber cords which are used as the reinforcements therefor.
As the performance of an automobile and road condition are improved, the driving speed is getting higher. Thus, lots of studies are made about a tire cord to be used as a reinforcement for a tire rubber so as to maintain the stability and durability of the tire even during a high speed driving.
A tire cord may be divided into multiple portions based on their functions, i.e., a carcass portion to support the entire tire, a belt portion to support the load and to prevent any deformation that might be caused when a high speed driving is made, and a cap ply portion to prevent any deformation of the belt portion. As the driving speed is getting higher due to the improvement of the highway conditions, the deformation of the belt portion of the tire cord occurs thereby causing the degradation of the riding quality. Thus, the cap ply portion to prevent the deformation of the belt portion is getting more important.
The materials mainly used now for the cap ply are nylon and aramid. The nylon are used for most kinds of the tires since it is less expensive and has higher adhesiveness both before and after fatigue compared to other materials. Furthermore, the nylon has high shrinkage stress which the cap ply is required to have to advantageously support the belt cord during a high speed driving. However, the nylon has drawbacks as a material for a cap ply in that a flat spot might be caused due to its low modulus and high changeability between room temperature and high temperature.
The aramid, one of the materials other than nylon for the cap ply, causes little flat spot problem, i.e., tire deformation after long-term parking, because, although having lower shrinkage stress than nylon, it has good creep property and very high modulus, and exhibits only small difference in modulus between room temperature and high temperature. While having been used for the high-class tires for which the quality of tire is very important, such aramid material cannot be used for the general tires as a practical matter because it is very expensive. Furthermore, since the high modulus of the aramid makes it difficult to expand the tire during the tire forming and curing processes, it is hard to apply the aramid material to the general tires. It has also a disadvantage in that its elongation at break is too low to secure sufficient fatigue resistance, i.e., long-term durability.
To compensate for the aforementioned drawbacks, a hybrid structure has been developed which comprises both nylon and aramid. In most hybrid structures, however, the primary twist numbers of the nylon and aramid primarily-twisted yarns and the secondary twist number of a ply yarn are different from one another because of the big difference between the physical properties of the nylon and aramid. Although such structure can solve the expansion-related problem during the tire-manufacturing process and the fatigue durability problem as well, since it requires the primarily-twisted yarns of different twist numbers as well as the ply yarn to be manufactured separately with different ring twisters or special twisters, it has such limitations as low productivity, high variability of the physical properties due to its unstable structure, high defect rate, and the like.
More particularly speaking, since the conventional hybrid fiber cord comprises the nylon and aramid primarily-twisted yarns having different twist numbers respectively and they are secondarily-twisted at further different twist number, the physical properties of the entire hybrid fiber cord cannot but be affected dominantly by the twist numbers of the primary and secondary twists.
According to the conventional method, as schematically illustrated in FIG. 1, when the nylon primarily-twisted yarn 11 and aramid primarily-twisted yarn 12 are secondarily twisted together to form the ply yarn 10, the nylon primarily-twisted yarn 11 is covered by the aramid primarily-twisted yarn 12. Thus, when the ply yarn 10 is dried and heat-treated after it is submerged into an adhesive solution, significant friction between the ply yarn 10 and the guides and rollers occurs thereby sweeping the aramid primarily-twisted yarn 12 which covers the nylon primarily-twisted yarn 11 to form a loop, and/or the nylon primarily-twisted yarn 11 shrinks causing the shape non-uniformity.
The loop and shape non-uniformity make the properties of the hybrid fiber cords non-uniform and cause defective products.